Further smooth "Add CC" interaction in Maniphest

Summary: Ref T2217. Fixes T3877. Improves more behaviors in the presence of "no effect" detection and new comment storage.

Test Plan: Added CCs, added existing CCs, implicitly added CCs via reassign, added CCs with comment.

Reviewers: btrahan, garoevans

Reviewed By: garoevans

CC: aran

Maniphest Tasks: T2217, T3877

Differential Revision: https://secure.phabricator.com/D7100